This invention relates to a flow, or rotary type of photographic recorder for optically recording the images of documents in greatly reduced size upon film while those documents are continuously transported past an aperture area, and relates specifically to an improved circuit for maintaining the illumination constant in the aperture area.
In a rotary recorder the images of documents are optically recorded on rolls of photographic film. The documents are caused to move at an essentially constant velocity through the photographic area which includes a fixed slit aperture and an illumination source. As each document moves through the recorder, it trips a control switch which starts the film moving and opens the camera shutter so that as the document passes the aperture the image of the illuminated document is reflected by a series of mirrors into the camera lens and onto the film which is moving at a speed proportional to the speed of the document. Because the movement of film and document is so proportioned, the image of the document appears stationary, or essentially so, on the surface of the film.
As the moving documents pass through the aperture area the only other variable is the illumination on the documents. Obviously if the illumination changes as a document moves past the aperture the brightness of the recorded image will change with it. Hence it is desirable to be able to adjust the illumination level for best recording and to thereafter maintain the illumination level for line voltage variations as well as lamp life. The problem of a constant illumination level is particularly complicated where a lamp driven by an alternating current source is chosen such as a fluorescent bulb. There, in order to regulate the alternating current source cross over distortion becomes a problem which introduces a flicker in the illumination and causes a line or dash spot to appear across the recording film.